1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof, and, more particularly to an image forming apparatus employing an electrophotographic system and a control method thereof.
2. Description of the Related Art
In an image forming apparatus such as a copying machine employing an electrophotographic system, a process for developing an electrostatic latent image formed on a photoconductive drum using a toner or the like, transferring a toner image developed onto a sheet, and then heating and pressing the toner image with a fixing device to fix the toner image on the sheet is adopted.
As a structure of the fixing device, a form of using a heating roller having a heater built therein is often adopted. The toner image on the sheet is fixed by directly applying heat of the heating roller to the sheet or indirectly applying the heat to the sheet via a fixing belt or the like.
In order to secure satisfactory fixing performance, it is important to maintain the temperature of the heating roller within a predetermined range.
However, when a large number of sheets are continuously printed, since heat absorbed by the sheets increases, the temperature of the heating roller falls.
On the other hand, since the heating roller is cold during starting of the image forming apparatus, a warm-up time for heating the heating roller using the heater to bring the temperature of the heating roller into the predetermined range is necessary.
In order to maintain the temperature of the heating roller within the predetermined range during continuous printing as well or reduce the warm-up time, techniques including a main heater and an auxiliary heater as heaters are disclosed in, for example, JP-A 2004-070011 and JP-A 2004-319431.
In these disclosed techniques, a commercial power supply is used as a power supply for the main heater and a chargeable auxiliary power supply, for example, an auxiliary power supply employing an electric double layer capacitor is used as a power supply for the auxiliary heater.
The techniques disclosed in JP-A 2004-070011 and JP-A 2004-319431 are forms of always using the main heater and the auxiliary heater complementarily in a state in which an auxiliary power supply for the auxiliary heater is always built in. Therefore, when sheets are subjected to the continuous printing at predetermined specified speed, it is possible to control the temperature of the heating roller to be within the predetermined range by using the auxiliary heater. However, when speed of the continuous printing is increased by specified speed, since the temperature of the heating roller further falls, fixing performance cannot be maintained.
As a general characteristic of a chargeable power supply, discharging electric power is smaller than charging electric power. Thus, power use efficiency inevitably falls in the form of always using both the auxiliary power supply and the commercial power supply complementarily compared with a form of using only the commercial power supply.